1. Technical Field
The present invention relates to a system for stirring by a magnetic coupled impeller, for use in pressure, volume and temperature (pVT) studies, of reservoir fluids and their properties in the laboratory and in the field.
In pVT cells and condensate cells, petroleum fluids can be studied at varying pressure and temperature simulating the conditions in oil reservoirs before and during production. Typically these fluids contain gas. The change in fluid density (compressibility) and the tendency for the gas to come out of the solution at decreasing pressure are of particular interest.
pVT cells are optimized to study oils with dissolved gas, while condensate cells are optimized to study light oils with high gas to oil ratio. In the following description they are both referred to as pVT cells.
Until the late eighties, the method to control the pressure in these cells was to pump mercury in and out of the cells. Mercury was considered to be inert with the respect to the petroleum fluids. There were however some health risks involved in the handling of mercury at high pressure and temperature, and mercury pumping has to a large extent been replaced by other methods for changing the volume in pVT cells.
Several of the new designs are based on cylindrical cells with a sealed piston that can be moved by either direct mechanical drive or hydraulic drive.
By shaking the old pVT cells with mercury, the mercury would also provide good stirring, so that a fast equilibrium between the phases was obtained. Equilibrium is essential to achieve reliable and reproducible measurements. This feature is lost when the volume is controlled with a piston.
2. Description of Related Art
As described in more detail later, several pVT cells are equipped with a magnetically coupled stirrer for mixing of the fluids under test.
The principle of a loose magnetic pulse driven stirrer and placement/stirrer speed/stirrer drag in a high pressure cells is, among other places, described in Norwegian patent no. 312.921. While patent 312.921 is an optical method to detect viscosity changes, the present invention is suitable for any fluid where optical methods are not suitable.
The magnetic coupling between the solenoids and the stirrer is relatively weak, due to the geometry and because the magnetic field is supplied through the metal piston.
Some petroleum fluids are very viscous, and the stirrer may therefore not rotate at the desired frequency.
Some cells are not provided with a window, and it is therefore not possible to see the fluid (blind cells). In many cases it would not be possible to see the stirrer even if the cell has a window, because the fluids are so dark.
A monitoring device to indicate whether the impeller is stirring or not, is therefore needed to achieve assurance of reliable operation.
Several patents, e.g. U.S. Pat. No. 6,834,990, describe a shaft driven stirrer with various configurations, especially of the stirrer/impeller with the focus on providing special features, e.g. aeration with bubbles from air supplied through the shaft and high efficiency (low power) shear of the fluid etc.
The shaft driven stirrers are not magnetically coupled, but directly driven, with related sealing problems. The magnetic coupling provides a possibility of a closed container with long time pressure stability.
One patent, U.S. Pat. No. 6,007,227, describes a control system for a blender application. A control system typically involves a sensor for feedback of the controlled variable. However, in the present invention, the stirrer acceleration, speed or angular position is not a part of a control system.
An alternative apparatus from those described above is needed to perform pVT studies with a simple and reliable assurance of stirring and therefore mixing of fluids or phase equilibrium under test in cells without a means for visual observation of the fluid, or for fluids with high level of opacity in cells with means for visual observation. The alternative for monitoring should also be very compact with no moving parts.